This is a patent application based on Japanese patent applications No. 2002-063811 and No. 2003-040462, which were filed on Mar. 8, 2002 and Feb. 19, 2003, respectively, and which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor laser. Especially, the present invention relates to a group III nitride compound semiconductor laser having a ridge type of a resonator.
2. Description of the Related Art
A semiconductor laser which has a cladding layer and an active layer and so on made of a group III nitride compound semiconductor (AlxGayIn1xe2x88x92xxe2x88x92yN, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and 0xe2x89xa6x+yxe2x89xa61) has been known. The conventional semiconductor laser is a semiconductor diode having multi-layer structure with multiple group III nitride compound semiconductor layers, which are formed on a sapphire substrate in sequence. A conventional example of the semiconductor diode, disclosed by the same applicant in Japanese Patent Laid-open No. 2000-261105, is shown in FIG. 3.
A semiconductor laser 900 shown in FIG. 3 has the following eight layers formed on a sapphire substrate 91 in sequence: a buffer layer 92; an n-layer 93; an n-cladding layer 94; an n-guide layer 95; an active layer 96 made of multiple quantum well (MQW) layer; a p-guide layer 97; a p-cladding layer 98; and a p-contact layer 99. As shown in FIG. 3, a cavity or a resonator part (a ridged resonator cavity of part) A is formed by using, e.g., photoresist and etching, and a positive electrode 901 and a negative electrode 902 are formed on the upper surface of the p-contact layer 99 and the etched surface of the n-layer 93, respectively.
The active layer 96, comprising a multiple quantum well (MQW) layer, is a semiconductor layer which functions as a main layer to oscillate laser. Each carriers (holes and electrons) injected from the positive electrode 901 and the negative electrode 902 combine in the active layer 96, that causes laser oscillation. The n-guide layer 95 and the p-guide layer 97 function to confine carriers into the active layer 96. Also, the n-cladding layer 94 and the p-cladding layer 98 function to confine laser light. And the n-layer 93 and the p-contact layer 99 are semiconductor layers which are formed in order that carriers can be injected smoothly from the negative electrode 902 and the positive electrode 901 to the layers existing between the n-cladding layer 94 and the p-cladding layer 98, respectively.
In order that the semiconductor laser made of group III nitride compound semiconductor can oscillate laser efficiently, the cross-section of electric current path of the semiconductor laser is, for example, narrowed by decreasing the contact area of electrodes, or by decreasing the width w of the positive electrode 901. In addition, the above-mentioned Japanese Patent Laid-open No. 2000-261105 suggests forming a deep ridged hole injection part B. That is, a boundary between a ridged cavity part A and the ridged hole injection part B is regarded as a boundary between the p-guide layer 97 and the p-cladding layer 98.
When forming the ridged hole injection part B, however, it is not easy for all the semiconductor lasers formed on a wafer that a boundary between the ridged resonator part A and the ridged hole injection part B functions as a boundary between the p-guide layer 97 and the p-cladding layer 98. The reason is that each one of group III nitride compound semiconductor layers formed on one wafer has different thickness according to the portion on which the layer is formed. So, as discloses in the above-mentioned official gazette, the applicant of the present invention suggests completely etching the p-cladding layer 98 even if a portion of the p-guide layer 97 is etched.
The thickness of the p-guide layer 97, however, is extremely thin, e.g.,. about 100 nm. So when 200 nm in thickness of p-contact layer 99 and approximately 500 nm in thickness of p-cladding layer 98 are completely etched, the p-guide layer 97 may be damaged considerably, which may deteriorate its device characteristic as a semiconductor laser.
An object of the present invention is to form a ridged carrier injection part in a ridge type of a group III nitride compound semiconductor laser, especially in a process of manufacturing the ridge type of a group III nitride compound semiconductor laser, so as to obtain a structure which hardly damages a guide layer. Another object of the present invention is that the cross sectional shape of oscillated laser beam becomes closer to a perfect circle by forming a part of cladding layer in the ridged cavity part and controlling its thickness becomes easy.
To achieve the above object, a first aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a laser cavity and multiple layers which are made of group III nitride compound semiconductors and formed on a substrate. The group III nitride compound semiconductor laser comprises: a first layer, which functions as a guide layer and actually confines carriers to an active layer which functions as a main layer oscillating laser; a second layer having smaller refractive index compared with the first layer, which is formed above or on the first layer and mainly confines light to the active layer and the first layer; and a third layer which is formed between the first layer and the second layer or formed into the second layer and has larger composition of aluminum (Al) in group III elements compared with the second layer. Here forming the third layer into the second layer represents that the second layer comprises two layers and that the third layer is formed between the upper second layer and the lower second layer. Composition of the upper second layer and the lower second layer may be equivalent or not equivalent. In order that the third layer comes in the scope of the present invention, aluminum (Al) composition of the third layer may be larger than that of at least one layer of upper and the lower second layers.
The second aspect of the present invention is that aluminum (Al) composition of group III elements in the second layer is larger than that in the first layer.
The third aspect of the present invention is that the second layer functions as a cladding layer.
The fourth aspect of the present invention is to obtain a group III nitride compound semiconductor laser comprising a laser cavity. The laser cavity is formed by removing multiple layers, which are made of group III nitride compound semiconductors and formed on a substrate, except the width of the laser cavity part. A carrier injection part is formed contacting to the laser cavity part by removing at least all layers on the third layer except the area corresponding to the width of an electrode formed above the second layer.
The fifth aspect of the present invention is that the electrode is a positive electrode.
The sixth aspect of the present invention is that aluminum (Al) composition of the third layer is larger than that of the second layer by 10% or more. That is represented by the formula x3xe2x89xa7x2+0.1 when x3 and x2 (0xe2x89xa6x3, x2xe2x89xa61) are aluminum (Al) compositions of the third and the second layers in all the group III elements. The seventh aspect of the present invention is that the third layer is thinner than the first layer.
By forming the third layer having larger aluminum (Al) composition in group III elements compared with that of the second layer between the first layer which functions as a guide layer and actually confines carriers to an active layer functioning as a main layer to oscillate laser and the second layer which mainly confine light to the active layer which functions as a main layer oscillating laser and the first layer, the third layer can protect the first layer in an etching process. That is because etching rate of a group III nitride compound semiconductor becomes smaller in proportion as the aluminum (Al) composition increases, there are some time during etching the third layer. Accordingly, in a process of forming a ridged carrier injection part, a guide layer cannot be damaged even when the production is not uniform. Or because the third layer having larger aluminum (Al) composition of group III elements compared with the second layer is formed between the upper second layer and the lower second layer, forming a part of a cladding layer in the laser cavity part enables it easier to control thickness of the cladding layer formed in the laser cavity part. As a result, the cross sectional shape of oscillated laser beam becomes closer to a perfect circle (the first to the fourth aspects). And when a layer of the positive electrode side, or the second layer, has p-type conduction, manufacturing process becomes easier (the fifth aspect).
Because the difference between the aluminum (Al) composition ratios of the second layer and the third layer is 10% or more, etching rate of the third layer becomes smaller and the etching becomes more effective (the sixth aspect). By forming the third layer thinner than the first layer, the guide layer may not be damaged without deteriorating characteristics of a laser diode or thickness of the cladding layer formed in the laser cavity part may be controlled easily (the seventh aspect).